The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Where sea-going vessels are designed for high speeds, it is generally desirable for reasons of efficiency to have a hull which is long and thin. However, this leads to a lack of stability and the vessel might easily capsize. Therefore, many high speed vessels are designed with two or more hulls which are long and thin, or long and narrow, and are arranged in such a way as to provide improved stability. Typical examples of such vessels are the two-hulled catamaran and the three hulled trimaran.
The long, thin hull realises the desire for propulsion efficiency with high speed for installed power, but long, thin hulls are easily affected by waves, and vessel motions can become undesirably excessive. The reason for this concerns the area and the moment of inertia of the waterplane of the vessel. A vessel with a high waterplane area and moment of inertia will resist motions more easily than one with a smaller waterplane area and moment of inertia.
A vessel having a relatively thin hull will have a smaller waterplane area and moment of inertia, and it follows that such a vessel will be more susceptible to wave-induced motion.
However, a vessel that is easily set into motion by waves is conversely easily restrained against such movement by the addition of surfaces mounted perpendicular to the direction of wave-induced motion, which is generally vertically.
It is known to configure a trimaran as a high-speed ferry having a long, thin main hull and two smaller long, thin side hulls (amahs), which provide the necessary stability to the whole vessel to prevent capsizing. Pitching motion of the trimaran may be controlled by the generation of vertical forces produced by an active hydrofoil mounted at the bow of the main hull, in a similar fashion to similar active hydrofoils mounted at the bows of various catamarans. To restrict the trimaran vessel from rolling in waves, anti-roll fins have been fitted on each side of the main hull. These anti-roll fins are essentially moving hydrofoils, similar to the corresponding devices fitted to more conventional monohull vessels.
Anti-roll fins are effective in reducing roll, but they have an unwanted side effect in that they will also generate forces in unwanted directions when the vessel is rolling; specifically, they can induce yaw, which will move the vessel off the selected course, as well as a small component of sway. The yawing and sway forces are the inevitable result of fins which are not horizontal, as the lift generated by a fixed non-horizontal fin when a vessel rolls will have vertical component (reducing rolling) and a horizontal component (increasing yaw and sway).
It is known that it is beneficial in order to reduce rolling to position the fins perpendicular to the line of rolling, which usually results in the anti-roll fins being mounted off the horizontal plane.
Any unwanted yaw forces will require a steering course-correction from the helmsman every time the vessel rolls, which is clearly undesirable.
It is against this background and the problems and difficulties associated therewith, that the present invention has been developed.